On Mon, May 13, 2019 at 11:57 AM Stanislav Fomichev <sdf@xxxxxxxxxxx> wrote:
>
> On 05/08, Stanislav Fomichev wrote:
> > On 05/08, Alexei Starovoitov wrote:
> > > On Wed, May 08, 2019 at 10:18:41AM -0700, Stanislav Fomichev wrote:
> > > > Right now we are not using rcu api correctly: we pass __rcu pointers
> > > > to bpf_prog_array_xyz routines but don't use rcu_dereference on them
> > > > (see bpf_prog_array_delete_safe and bpf_prog_array_copy in particular).
> > > > Instead of sprinkling rcu_dereferences, let's just get rid of those
> > > > __rcu annotations and move rcu handling to a higher level.
> > > >
> > > > It looks like all those routines are called from the rcu update
> > > > side and we can use simple rcu_dereference_protected to get a
> > > > reference that is valid as long as we hold a mutex (i.e. no other
> > > > updater can change the pointer, no need for rcu read section and
> > > > there should not be a use-after-free problem).
> > > >
> > > > To be fair, there is currently no issue with the existing approach
> > > > since the calls are mutex-protected, pointer values don't change,
> > > > __rcu annotations are ignored. But it's still nice to use proper api.
> > > >
> > > > The series fixes the following sparse warnings:
> > >
> > > Absolutely not.
> > > please fix it properly.
> > > Removing annotations is not a fix.
> > I'm fixing it properly by removing the annotations and moving lifetime
> > management to the upper layer. See commits 2-4 where I fix the users, the
> > first patch is just the "preparation".
> >
> > The users are supposed to do:
> >
> > mutex_lock(&x);
> > p = rcu_dereference_protected(prog_array, lockdep_is_held(&x))
> > // ...
> > // call bpf_prog_array helpers while mutex guarantees that
> > // the object referenced by p is valid (i.e. no need for bpf_prog_array
> > // helpers to care about rcu lifetime)
> > // ...
> > mutex_unlock(&x);
> >
> > What am I missing here?
>
> Just to give you my perspective on why current api with __rcu annotations
> is working, but not optimal (even if used from the rcu read section).
>
> Sample code:
>
> struct bpf_prog_array __rcu *progs = <comes from somewhere>;
> int n;
>
> rcu_read_lock();
> n = bpf_prog_array_length(progs);
> if (n > 0) {
> // do something with __rcu progs
> do_something(progs);
> }
> rcu_read_unlock();
>
> Since progs is __rcu annotated, do_something() would need to do
> rcu_dereference again and it might get a different value compared to
> whatever bpf_prog_array_free got while doing its dereference.
correct and I believe the code deals with it fine.
cnt could be different between two calls.
> A better way is not to deal with rcu inside those helpers and let
> higher layers do that:
>
> struct bpf_prog_array __rcu *progs = <comes from somewhere>;
> struct bpf_prog_array *p;
> int n;
>
> rcu_read_lock();
> p = rcu_dereference(p);
> n = bpf_prog_array_length(p);
> if (n > 0) {
> do_something(p); // do_something sees the same p as bpf_prog_array_length
> }
> rcu_read_unlock();
>
> What do you think?
I'm not sure that generically applicable.
Which piece of code do you have in mind for such refactoring?
